This invention relates to display technology, and more particularly to the fabrication of an array of atomically sharp field emission tips.
The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness. The process of the present invention is directed toward the fabrication of very sharp cathode emitter tips.
A great deal of work has been done in the area of cold cathode tip formation. See, for example, the xe2x80x9cSpindtxe2x80x9d patents, U.S. Pat. Nos. 3,665,241, and 3,755,704, and 3,812,559 and 5,064,396. See also, U.S. Pat. No. 4,766,340 entitled, xe2x80x9cSemiconductor Device having a Cold Cathode,xe2x80x9d and U.S. Pat. No. 4,940,916 entitled, xe2x80x9cElectron Source with Micropoint Emissive Cathodes and Display Means by Cathodeluminescence Excited by Field Emission Using Said Source.xe2x80x9d
One current approach toward the creation of an array of emitter tips, is to use a mask and to etch silicon to form a tip structure, but not to completely form the tip. Prior to completing a sharp point, the etching process is discontinued. The idea is to catch the etch at a stage before the mask is dislodged from the apex of the tip. See, for example, U.S. Pat. No. 5,201,992 to Marcus et al., entitled, xe2x80x9cMethod for Making Tapered Microminiature Silicon Structures.xe2x80x9d
Prior art teaches that it is necessary to terminate the etch at or before the mask is fully undercut to prevent the mask from being dislodged from the apex. If an etch proceeds under such circumstances, the tips become lop-sided and uneven due to the presence of the mask material along the side of the tip, or the substrate during a dry etch and additionally, the apex may be degraded, as seen in FIG. 8. Such a condition also leads to contamination problems because of the mask material randomly lying about a substrate, which will mask off regions where no masking is desirable, and continued etching will yield randomly placed, undesired structures in the material being etched.
If the etch is continued, after the mask is removed, the tip may become more dull. This results because the etch chemicals will remove material in all directions, thereby attacking the exposed apex of the tip while etching the sides. In addition, the apex of the tip may be degraded when the mask has been dislodged due to physical ion bombardment during a dry etch.
Hence, the tendency is to underetch (i.e, stop the etch process before a fine point is formed at the apex of the tip) the tip, thereby creating a structure referred to as a xe2x80x9cflat top.xe2x80x9d Then, an oxidation step is typically performed to sharpen the tip. This method results in a non-uniform etch results across the array, and the tips will have different heights and shapes.
Others have tried to manufacture tips by etching, but they do not undercut the mask all the way as in the process of the present invention, and furthermore do not continue etching beyond full undercut of the mask without suffering degradation to the tip as in the process of the present invention, which allows for latitude which is required for manufacturing. Rather they remove the mask before the tip is completely undercut, and sharpen the tips from there. The wet silicon etch methods of the prior art, result in the mask being dislodged from the apex of the tip, at the point of full undercut which can contaminate the etch bath, generate false masking, and degrade the apex.
The non-uniformity among the tips may also present difficulties in subsequent manufacturing steps used in the formation of the display, especially those processes employing chemical mechanical planarization. See for example, U.S. Pat. No. 5,229,331, entitled, xe2x80x9cMethod to Form Self-Aligned Gate Structures Around Cold Cathode Emitter Tips Using Chemical Mechanical Polishing Technology,xe2x80x9d and U.S. Pat. No. 5,186,670, entitled, xe2x80x9cMethod to Form Self-Aligned Gate and Focus Rings,xe2x80x9d also assigned to Micron Technology, Inc. Non-uniformity is particularly troublesome if it is abrupt, as opposed to a gradual change across the wafer.
Fabrication of a uniform array of tips using current processes is very difficult to accomplish in a manufacturing environment for a number of reasons. For example, simple etch variability across a wafer will effect the time at which the etch should be terminated with the prior art approach.
Generally, it is difficult to attain plasma tip etches with uniformities better than 5%, with uniformities of 10%-20% being more common. This makes the xe2x80x9cflat topxe2x80x9d of an emitter tip etched using conventional methods vary in size. In addition, the oxidation necessary to xe2x80x9csharpenxe2x80x9d or point the tip varies by as much as 20%, thereby increasing the possibility of non-uniformity among the various tips of an array.
Tip height and other critical dimensions suffer from the same effects on uniformity. Variations in the masking uniformity, and material to be etched compound the problems of etch uniformity.
Manufacturing environments require processes that produce substantially uniform and stable results. In the manufacture of an array of emitter tips, the tips should be of uniform height, aspect ratio, sharpness, and general shape, with minimum deviation, particularly in the uppermost portion.
The process of the present invention employs dry etching (also referred to as plasma etching) to fabricate sharp emitter tips. Plasma etching is the selective removal of material through the use of etching gases. It is a chemical process which uses plasma energy to drive the reaction. Those factors which control the precision of the etch include the temperature of the substrate, the time of immersion, the composition of the gaseous etchant, pressure, applied RF power, and etch hardware configuration.
The mask layer is formed such that it exposes the silicon substrate, which silicon substrate is then etched to form the sharp emitter tips.
The process of the present invention can be used to produce sharp tips with relatively any given aspect ratio and height with a single step (in situ) or multi-step plasma dry etch process.
The present invention, under some conditions provides a very large manufacturing window, particularly when the tips are etched into a layer or substrate in which the thickness of the layer is not totally consumed during the tip etch in unmasked (i.e., non-tip) regions.
In the preferred embodiment, a dry etch proceeds for about 2.3 minutes to undercut the mask and form a sharp tip. An overetch can continue the process without a substantial change in the appearance of the tips. The shape of the tip is self-repeating because the mask has been optimized to remain in place relative to the top of the emissive structure region being formed. The tip is etched vertically, as well as horizontally, and the shapes are most uniform in appearance when the rate of horizontal etching is within a factor of four to the vertical, with the most uniform results occurring after a 2:1 ratio of vertical to horizontal etching rate.
Contrary to the current teaching, the present invention involves dry etching the apex of the tip to a complete point, and continuing etching to add the requirement of process margin required in manufacturing, such that the mask appears as a see-saw or teeter-totter at equilibrium, essentially perfectly balanced on the apex of the tip.
In the preferred embodiment, a substrate of 14-21 ohms-cms P-type 1-0-0 single crystal silicon is the material from which the tips are formed. The mask in the preferred embodiment has a circular shape, and is comprised of 0.1 xcexcm thick thermal silicon dioxide with a diameter of 1 xcexcm. Contrary to prior art teachings, the mask can be comprised of dimensions, and material selection, such that a particular etch process of a particular material may be employed with that mask, and the mask will adhere to the tip and can be overetched, beyond full undercut without adversely effecting tip shape and uniformities.
This benefit is believed to be obtained as a result of the attractive forces between the mask and the tip, such as Vander Waals, electrostatic, and electrochemical forces.
Experiments were undertaken with a variety of masks, having differing compositions and dimensions in combination with the etch conditions of the Table 1 below, and a tip material of 14-21 ohm-cm 100 p-type single crystal silicon. The mask formed from a layer of 1 xcexcm. thick HPR 6512 photoresist (Hunt Photoresist), and 0.1 xcexcm. thick thermal silicon dioxide stack, was found to be unsatisfactory for use in the present invention. It became dislodged from the tips during the etch process, resulting in malformed tips. This effect is believed to be influenced by the mass of the etch mask.
Other masks which were found to be unsatisfactory for use in the present invention include: a 0.4 xcexcm. oxide mask; and a 1 xcexcm. mask comprised solely of HPR 6512 photoresist.
However, a mask comprising 0.1 m. thick thermal oxide has displayed very good results in the present invention, as well as a mask of 0.05 xcexcm. thick thermal oxide.
One advantage of the process of the present invention is that it enables the fabrication of tips having more uniform distribution of tip dimensions. Another advantage is that it enables the formation of a good distribution of extremely sharp points which may be enhanced by further processing, but are enabled functional with etching as a tip formation only. Yet still another advantage is that it provides a method for overetching with a dry etch without significantly degrading the desired tip shape.
One aspect of the process of the present invention involves a method for fabricating sharp asperities. A substrate is provided which has a mask layer disposed thereon, and a layer of beads or micro-spheres is disposed superjacent the mask layer. The beads or micro-spheres are for patterning the mask layer. The substrate is etched, thereby creating sharp asperities.
Another aspect of the process of the present invention involves a masking process useful in etching micro-tips. An oxide layer is formed superjacent a substrate. The oxide layer is patterned using synthetic beads, and then the oxide layer is etched.
Yet another aspect of the process of the present invention involves a patterning process useful for fabricating micro-tips. A first layer is formed superjacent a substrate, and a layer of beads or micro-spheres is placed upon the first layer. The first layer is etched, and the substrate is also etched, thereby fabricating micro-tips.
As one point becomes sharp, it continues to etch for a period of time, with the mask xe2x80x9cfollowingxe2x80x9d the tip down as small amounts of material are removed from the very apex of the tip, as etching continues beyond full undercut of the mask. For this reason, once an emitter tip is etched to a point, its dimensions become fixed. All tips on a substrate continue to etch until they become sharp, at which point, they have substantially the same height, aspect ratio, and sharpness.
Oxidation of tips can be employed to provide sharper emitters with lower electric fields required to produce emission, the benefits of oxidation sharpening are more controlled and a more efficiently exploited with the tip etch of the present invention, since the tip geometry is maintained rather than altered.